The present invention relates generally to multi-split air conditioning systems, and is directed more particularly to an improved interface circuit for use in such systems.
A typical air conditioning system includes an indoor unit having an evaporator coil, a fan and a control circuit for turning the fan on and off in accordance with the temperature of the indoor space being conditioned. This indoor unit operates in conjunction with an outdoor unit including a compressor, a condenser, a fan and a control circuit for turning one or more of the latter on and off in accordance with the cooling requirements of the indoor unit. These cooling requirements are typically communicated from the indoor unit to the outdoor unit over a pair of control wires which transmit a control voltage produced by the indoor unit. This control voltage is derived from the AC source which drives the fan, and is transmitted as a result of the closure of a contact by the indoor control circuit.
In air conditioning systems which include only a single indoor unit and a single outdoor unit, the communication of control signals from the former to the latter is clear and unambiguous. The presence or absence of a control voltage conveys all of the information necessary to control the outdoor unit.
A more complex control situation exists in multi-split air conditioning systems which include a plurality of identical indoor units and a single outdoor unit with multiple control inputs. In such systems, the condenser and fan of the outdoor unit may be controlled in accordance with the total number of indoor units which are transmitting control signals thereto, without taking into account which control input of the outdoor unit is receiving which control signal.
A much more complex control situation exists in multi-split air conditioning systems which include a plurality of non-identical indoor units and a single outdoor unit. In such systems, the indoor units have different fan power requirements and, consequently, operate on AC voltages having different magnitudes. A small indoor unit, for example, may use an AC voltage as low as 24 volts, while larger indoor units may use 115 volts single phase. Still larger units may use 230 volts three phase in either a Wye or a Delta configuration. As a result, unless the control scheme of the system is to be changed to fit each user's application, the outdoor unit must be able to operate with control signals having any one of a plurality of different control voltage values. Thus, prior to the present invention, there has existed a need for a simple, cost effective way of enabling the control circuitry of outdoor units to accept control voltages of any of a variety of different magnitudes.